In order to transmit a high frequency signal, a coaxial cable is generally employed, which comprises a central conductor composed of a single conducting wire, an insulator made of Teflon, phenol resin or the like, surrounding the central conductor, a mesh outer conductor surrounding the insulator and an insulator outer skin.
Characteristic impedance of this coaxial cable is defined by the diameter of the central conductor and the dielectric constant of the insulator located between the central conductor and the outer conductor. The characteristic impedance of a general coaxial connectors is classified roughly into two categories, namely 50 .OMEGA. systems and 75 .OMEGA. systems.
When above described coaxial cables are connected with each other, in general, a coaxial connector is employed, which is composed of a plug arranged on each end and a jack. With respect to one example thereof, a general purpose plug 2, as shown in FIG. 9, has a plug body A with a cylindrical shape and is composed of a conductor. One end of the plug body A is formed as an inlet B to be conductively connected to a jack (not illustrated).
Further, in the plug 1 shown in FIG. 9, a clamp C projects from the other end of the plug body A in a direction opposite of inlet B. The clamp C is caulked (or soldered in some cases) to the outer conductor, so that plug body A can be fixably attached to one end of a coaxial cable D. A connection terminal E (contact pin) is fastened to one end of the plug body A via the insulator so as to project from the end of plug 1. Further, a cap F is connected to the end of the plug body A opposite of connection terminal E to surround the clamp C.
The plug 1 may be plugged into a jack which is fastened to the chassis of electrical equipment such as a TV receiver or a video cassette recorder, or a jack positioned on the end of another coaxial cable so as to connect the two. When plugging in plug 2, the connection terminal E is inserted within a jack and the inlet B of the plug body is arranged so as to be fitted in the outer periphery of the conducting member of the jack. Such a jack would have a similar configuration as that of plug 1, except that it would have a shape suitable to accept insertion of connection terminal E and inlet B of the plug 1.
As described above, both the plug and the jack of the conventional coaxial connector are soldered to a connection terminal (contact pin), wherein the central conductor of the coaxial cable D is thicker than the connection terminal itself, so that the connection terminals are conductively connected with each other.
The above arrangement is also employed in a coaxial connector for high frequency. The process of making such a coaxial connector is shown in FIGS. 10(a) through 10(f). For example, as shown in FIG. 10(a), an outer insulator skin 3 of the coaxial cable D is cut and a turnbuckle 12, a bushing 13, a band 14 are inserted therein. As shown in FIG. 10(b), after a clamp 6 is inserted between the outer conductor 4 and the outer insulator skin 3, then the outer conductor 4 is folded back to define a gap region between outer conductor 4 and insulator 5, and a collar 7 is inserted into this gap.
Next, as shown in FIG. 10(c), the outer conductor 4 is stripped away with a cutter at the front end of clamp 6 on the border of collar 7 and the insulator 5 is stripped away from the front end of collar 7 without damaging the central conductor 8. Then, as shown in FIG. 10(d), the arrangement in FIG. 10(c) is inserted into a first cylindrical body 9 of the conductor, in which a connection terminal 16 is arranged via an insulator 15 (which may be made of a phenol resin).
Engaging a slit of the first cylindrical body 9 and a slit of the clamp 6, the outer conductor 4 is soldered to the portion of the first cylindrical body 9 represented by reference numeral Hi, and a front end of the central conductor 8 and a connection terminal 16 are soldered (a portion represented by a reference numeral H2) by inserting a soldering bit from a window portion of the first cylindrical body 9. Next, as shown in FIG. 10(e), the band 14 is caulked to a collar of the clamp 6. Finally, as shown in FIG. 10(f), the jack is inserted into a second cylindrical body 18 of the conductor and fastened thereto by the turnbuckle 12, so that the bushing 13 maintains the band 14 in contact and pressed against the outer conductor 4.
As demonstrated in the above examples, all of the conventional coaxial connectors are connected with each other such that the central conductors 8 are connected via connection terminals (contact pins). Therefore, it is necessary to solder the central conductors 8 to the connection terminals through windows located in the plug and the jack. This work is very troublesome, and is problematic in that the insulator 15 may possibly be melted by the heat produced by soldering, thereby cause a short circuit between the outer conductor 4 and the central conductor 8.
Further, as the manufacturing and wire connecting process of the plug and jack are difficult and time consuming, the cost of manufacturing is increased.
As described above, the characteristic impedance of the coaxial cable D is determined by the diameter of the central conductor 8, the inside diameter of the outer conductor 4 and the dielectric constant of the insulator 5 therebetween. Conventionally, since the connection terminal 16 interposes in the coaxial connector, as a wire connection means of the central conductor 8, the portion where the connection terminal 16 interposes is thicker, and there is a region (air region having a dielectric constant which is different from that of the insulator 5) without insulators around the connection terminal 16. Due to this, it is necessary to enlarge the diameter of the coaxial connector to provide correct matching of the characteristic impedance.
In the case of a transmission path of a high frequency signal, when the impedance passes through different channel paths, reflection results. In order to increase transmission efficiency, it is necessary to prevent this reflection as much as possible. Since the coaxial cable is a link in the channel path, it is important that the connector itself be designed so as to have an identical characteristic impedance as that of the coaxial cable (typically, 50 .OMEGA. or 75 .OMEGA.).
In other words, matching of the characteristic impedance of the coaxial cable and coaxial connector is required to meet the above objectives. However, in conventional coaxial connectors, ideal impedance matching between coaxial connectors and coaxial cables has not yet been achieved.